This invention relates generally to complementary direct-coupled amplifiers and in particular to short circuit protection and idle current control for direct-coupled amplifiers.
Full complementary direct-coupled audio amplifiers are capable of delivering power into a load with little distortion. The output power is normally provided by complementary pairs of a class B amplifier with each pair conducting for half of a cycle. To minimize cross-over distortion as the output signal polarity changes, a bias or idle current is made to flow through the output transistor circuitry. This current is normally set by the band-gap voltage drops of diodes and transistor junctions forcing the idle current to flow. Since the band-gap of all junctions are temperature dependent and since the output transistors heat up during operation, the voltage across their junctions cause the idle current to increase and cross-over distortion to increase. When the output current increases, transistor junction temperature increases even more, the effect becomes cumulative and idle current reaches the point where without current limiting output transistors are damaged.
Additionally, amplifier output loads such as a speaker can change impedance or at times short out forcing high current to flow through the output transistors causing them to become damaged. This potential problem is normally prevented by providing short circuit protection such as shown in the complementary direct-coupled amplifier circuit of Application Note No. AN-485 of Motorola Semiconductor Products Inc., Phoenix, Ariz.; printed in U.S. 6-69. The circuit however, requires a large number of parts and forms a secondary feedback loop within the amplifer which tends to cause instability problems unless extreme care is used in the circuitry layout.
In U.S. Pat. No. 3,925,709, another circuit is found which provides over-current protection by utilization of a light emitting diode and a photodiode within an optical isolator. This structure, however, does not direct itself to the draining of current by the photo detector but only provides for actuation of a current regulating system responsive to the light emitting diode actuation.
The use of active feedback to improve differential amplifier performance featuring both hard wire and optical coupling is disclosed in inventor's previous U.S. Pat. No. 3,995,149. The patent, however, does not disclose where the optical devices must be connected in a complementary direct-coupled amplifier for providing short circuit and current limiting.